In recent years, with the realization of electronic devices having a smaller size and higher performance, there has been a need for a higher density of printed circuit boards. In such a printed circuit board having a higher density, as a conductive pattern is miniaturized, the conductive pattern tends to be separated from a base film. In view of this, as a substrate for a printed circuit board that meets such a need for a higher density, there has been a demand for a substrate for a printed circuit board, the substrate having good adhesiveness between a conductive layer and a base film.
In order to meet the demand, a substrate for a printed circuit board has been proposed in which a thin copper layer is formed on a heat-resistant insulating base film without providing an adhesive layer therebetween (refer to PTL 1). In this existing substrate for a printed circuit board, a thin copper layer having a thickness of 0.25 to 0.30 μm is formed on each of two surfaces of a heat-resistant insulating base film by sputtering, and a thick copper layer is formed on each of the thin copper layers by electroplating.